Cross-axial sensor for measuring magnetic fields emanating from magnetoelastic shafts

ABSTRACT

A sensor for measuring divergent magnetic fields emanating from a rotatable magnetoelastic shaft without directly contacting the shaft comprises diametrically opposed sensing elements held in spaced relation to the shaft. Each sensing element is adjacent a corresponding one of the magnetic zones and on opposite sides of the shaft so as to create a cross-axial sensor arrangement.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. patent applicationSer. No. 10/193,754, filed on Jul. 11, 2002, the description of which isincorporated herein by reference.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates in general to measuring and testing andmore particularly relates to an apparatus for measuring magnetic fields.Most particularly, the invention relates to a sensor for measuringdivergent magnetic fields emanating from a magnetoelastic shaft.

[0004] 2. Description of the Prior Art

[0005] Sensors for measuring divergent magnetic fields emanating from amagnetoelastic shaft are well known. Such sensors are commonly comprisedof four sensor pick-ups. Two pick-ups are provided in two magneticzones. All four pick-ups are required to obtain necessary rotationalvariation levels and a common mode rejection of uniform magnetic fields,such as the earth's magnetic field.

[0006] What is needed is a sensor that reduces the number of sensorpick-ups required, which, in turn, reduces cost and complexity of thesensor.

SUMMARY OF INVENTION

[0007] Generally speaking, the present invention is directed towards asensor that meets the foregoing needs. The sensor measures divergentmagnetic fields emanating from a rotatable magnetoelastic shaft withoutdirectly contacting the shaft. The sensor comprises diametricallyopposed sensing elements held in spaced relation to the shaft. Eachsensing element is adjacent a corresponding one of the magnetic zonesand on opposite sides of the shaft so as to create a cross-axial sensorarrangement.

[0008] Various objects and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the preferred embodiment, when read in light of theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0009]FIG. 1 is a diagrammatic representation of a dual axial sensor formeasuring divergent magnetic fields emanating from a magnetoelasticshaft.

[0010]FIG. 2 is a diagrammatic representation of a cross-axial sensor.

[0011]FIG. 3 is a diagrammatic representation of a dual cross-axialsensor.

DETAILED DESCRIPTION

[0012] Referring now to the drawings, wherein like numerals designatelike components throughout all of the several Figures, there isillustrated in FIG. 1 a sensor 10 a on a rotatable shaft 12 formeasuring divergent magnetic fields B1, B2 emanating from the shaft 12without directly contacting the shaft 12. The shaft 12 can be made of amagnetic alloy, wherein the magnetic alloy is a material component ofthe shaft 12, or carry a magnetic alloy layer on its outer peripheralsurface. When a torque is transmitted to the shaft 12, the magneticalloy is mechanically stressed or otherwise deformed. This causesmagnetic fields B1, B2 to emanate from the magnetic alloy. A componentof the magnetic field B1, B2 is sensed by the sensor 10 a to produce anoutput signal that correlates to a direction and magnitude of the torquetransmitted to the shaft 12.

[0013] The sensor 10 a illustrated in FIG. 1 has two sensing or pick-upelements, each of which is preferably in the form of a coil pair 14, 16.The coil pairs 14, 16 are held in spaced relation to the shaft 12. Thiscan be accomplished with any suitable support. As illustrated in FIG. 1,each coil pair 14, 16 includes two corresponding axially arranged coils14 a, 14 b and 16 a, 16 b that are in the same plane P1, P2. The coils14 a, 14 b and 16 a, 16 b of each coil pair 14, 16 are arranged in anopposite sensing configuration, as indicated by the directional arrowsadjacent the coils 14 a, 14 b, and 16 a, 16 b when viewing FIG. 1. Thecoils 14 a, 14 b and 16 a, 16 b are also in equal and opposite magneticfields B1, B2, which emanate from corresponding magnetic zones 12 a, 12b of the shaft 12. This cancels out uniform magnetic fields, such as theearth's magnetic field, which are common to the coils 14 a, 14 b and 16a, 16 b. The coil pairs 14, 16 are also diametrically opposed (i.e.,placed 180-degrees apart around the shaft 12) in order to cancel theundesirable effects, such as gain and offset changes due to relativeradial position changes between the shaft 12 and coil pairs 14, 16.These effects require the arrangement of four coils 14 a, 14 b and 16 a,16 b, two coils 14 a, 16 a and 14 b, 16 b adjacent each respectivemagnetic zone 12 a, 12 b, resulting in a dual axial sensor arrangement.

[0014] If there is a reasonable correlation between the two adjacentmagnetic zones 12 a, 12 b, one of the diametrically opposed coils 16 aadjacent the same magnetic zone 12 a can be axially displaced into theadjacent magnetic zone 12 b, as shown in FIG. 2. This creates across-axial sensor arrangement, as generally indicated at 10 b in FIG.2. In this arrangement, there is one coil 14 a, 16 a adjacent eachrespective magnetic zone 12 a, 12 b and the coils 14 a, 16 a are inclose parallel planes P1, P2. This allows the two relatively equal andopposite magnetic fields B1, B2 to be measured by the two coils 14 a, 16a, which are in opposite sensing configurations, as indicated by thedirectional arrows adjacent the coils 14 a, 16 a when viewing FIG. 2,while canceling uniform magnetic fields. In addition, the coils 14 a, 16a are on opposite sides of the shaft 12, or spaced circumferentially180-degrees apart. This allows an averaging of a periodic signal,causing a reduction in rotational variation while still compensating forthe radial relative motions of the shaft 12 with respect to thelocations of the coils 14 a, 16 a. Again, this assumes a closecorrelation between the two magnetic zones 12 a, 12 b. Since themagnetic zones 12 a, 12 b are adjacent to each other and reside onportions of the shaft 12 that have gone through identical forming,heat-treating, machining, and magnetizing processes, the correlationshould be very good.

[0015] Unless there is perfect correlation between magnetic zones 12 a,12 b, there is, of course, the potential for an increase in rotationalerror. Also, there can be a reduction in uniform field cancellation dueto the additional displacement between the parallel planes P1, P2 inwhich the coils 14 a, 16 a now reside. These two effects have proven tobe reasonable from a signal-to-noise perspective. However, since therotational error and uniform field cancellation can be cancelled out bythe physical averaging and placing of the coils, a mathematicalaveraging can be performed on two separate coil pairs 14 a, 14 b and 16a, 16 b in a dual cross-axial sensor arrangement, as generally indicatedat 10 c in FIG. 3. One coil pair 14 a, 14 b is disposed 180-degrees fromthe other coil pair 16 a, 16 b. Through mathematical averaging, thesignal performance (i.e., the uniform field cancellation and rotationalsignal reduction) can be regenerated with the same number of coils 14 a,14 b and 16 a, 16 b but two separate channels or redundant outputs 14 c,16 c. Each cross-axial pair 14 a, 14 b and 16 a, 16 b may have slightlyless performance but the final averaged signal should be substantiallyidentical to having four elements in combination, as illustrated inFIG. 1. This higher performance signal, instead of the individualsignals, can now be used for control purposes. Moreover, this allowssafety critical applications to have redundant pairs 14 a, 14 b and 16a, 16 b without the loss of performance.

[0016] While this invention has been described with respect to severalpreferred embodiments, various modifications and additions will becomeapparent to persons of ordinary skill in the art. All such variations,modifications, and variations are intended to be encompassed within thescope of this patent, which is limited only by the claims appendedhereto.

[0017] In accordance with the provisions of the patent statutes, theprinciple and mode of operation of this invention have been explainedand illustrated in its preferred embodiments. However, it must beunderstood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its spiritor scope.

What is claimed is:
 1. In combination: a shaft; and a sensor formeasuring divergent magnetic fields emanating from magnetic zones on theshaft, the sensor comprising: diametrically opposed sensing elementsheld in spaced relation to the shaft, each sensing element beingadjacent a corresponding one of the magnetic zones and on opposite sidesof the shaft so as to create a cross-axial sensor arrangement.
 2. Thesensor according to claim 1 wherein the sensing elements are coils. 3.The sensor according to claim 1 wherein the sensing elements are inclose parallel planes.
 4. The sensor according to claim 1 wherein thesensing elements are in opposite sensing configurations so as to canceluniform magnetic fields.
 5. The sensor according to claim 1 wherein themagnetic zones reside on portions of the shaft which have gone throughsubstantially identical forming, heat-treating, machining, andmagnetizing processes.
 6. The sensor according to claim 1 wherein thesensing elements are coil pairs held in spaced relation to the shaft andin a dual cross-axial sensor arrangement, a first one of the coil pairsbeing disposed 180-degrees from a second one of the coil pairs.
 7. Thesensor according to claim 1 wherein the two coil pairs have separateoutputs.
 8. The sensor according to claim 7 further comprising means formathematically averaging the outputs.
 9. In combination: a shaft; and asensor for measuring two relatively equal and opposite magnetic fieldsemanating from adjacent magnetic zones on the shaft, the sensorcomprising: a pair of diametrically opposed coils held in spacedrelation to the shaft, a first one of the coils being adjacent a firstone of the magnetic zones and a second one of the coils being adjacent asecond one of the magnetic zones and the coils are on opposite sides ofthe shaft so as to create a cross-axial sensor arrangement.
 10. Thesensor according to claim 9 wherein the coils are in close parallelplanes.
 11. The sensor according to claim 9 wherein the coils are inopposite sensing configurations so as to cancel uniform magnetic fields.12. The sensor according to claim 9 wherein the magnetic zones reside onportions of the shaft which have gone through substantially identicalforming, heat-treating, machining, and magnetizing processes.
 13. Incombination: a shaft; and a sensor for measuring two relatively equaland opposite magnetic fields emanating from adjacent magnetic zones onthe shaft, the sensor comprising: two coil pairs held in spaced relationto the shaft and in a dual cross-axial sensor arrangement, a first oneof the coil pairs being disposed 180-degrees from a second one of thecoil pair,
 14. The sensor according to claim 13 wherein the two coilpairs have separate outputs.
 15. The sensor according to claim 13further comprising means for mathematically averaging the outputs.